The formation of particles from viscous materials is well known. Conventional methods and apparatus often involve the formation of liquid portions or droplets which are subsequently collected and solidified. For example, Froeschke, U.S. Pat. No. 4,279,579, discloses an apparatus for the extrusion of a flowable mass onto a conveyor. The apparatus has inner and outer cylindrical coaxial containers. The inner container, positioned within the inner container, has a passage for dispensing the flowable mass. The outer container has a number of orifices and rotates around the inner container. As the outer container rotates, the orifices on the outer container cyclically align with the passageway on the inner container. With each alignment, the flowable mass flows from the inner container, through the aligned orifices, and is apportioned and deposited on a conveyor, for example a conveyor belt, to form what is often referred to as pastilles.
Chang et al., U.S. Pat. No. 5,340,509, discloses a pastillation process for pelletizing ultra high melt flow crystalline polymers, i.e., a crystalline polymer which is a polyolefin homopolymer, a polyolefin copolymer, or blends thereof. Initially, molten polymer is transferred to a droplet-forming means. The droplet-forming means is generally an outer container, with orifices, which rotates around an inner container to allow a uniform amount of the polymer melt to emerge as droplets. The droplets are collected on a conveyor, which cools the droplets for a time sufficient to solidify the droplets.
Forming robust, uniform, pellets of a polyester material has been difficult or problematic. For example, low molecular weight polyesters, characterized as oligomers or prepolymers, may have such a low viscosity that initial particle formation may be difficult. The oligomer may be too liquid to form particles or pellets of uniform shape and size. This is because oligomers, having relatively short chain length, may have a relatively low amount of chain entanglement, in addition to limited intermolecular bonding or forces.
Known processes for forming polyester particles may result in particles which lack structural integrity. The weakness of such particles may make them hard to handle and susceptible to abrasion during transport or other mechanical handling. Abrasion may generate undesirable amounts of fines.
Polyester particles are useful as feedstock to a process to produce a higher molecular weight polymer, including solid-phase ("solid-state") polymerization processes. For such processes, it is desirable that the particles have certain characteristics. For example, particles having relatively uniform size and shape, for uniform polymerization within each particle, may be desirable. For solid-state polymerization, it is desirable that the particles be sufficiently robust to withstand the high temperatures of solid-state polymerization without agglomerating.
Conventionally, robust particles of polyester may be obtained by subjecting the particles to a lengthy and expensive heat treatment or annealing step. Such annealing increases the crystallinity and robustness of the particles. Such annealing, however, typically adds time and expense to an overall process for producing high molecular weight product. It would be desirable to reduce or eliminate such annealing.
In view of the above, there exists a need for an improved process and apparatus for the formation of polyester particles. There is a need for the more economical and efficient production of quality polyester particles, which, for example, are useful under rigorous circumstances and with limited pre-treatment prior to use as feedstock for further polymerization. Furthermore, there exists a need for an improved process of forming a low molecular weight polyester oligomer into crystalline particles. In addition, it would be a further advantage if the resulting particles exhibited improved crystalline morphology or related properties compared to conventional processes.